A filter is an apparatus for letting only signals of specific frequency bands pass (filtering), and is implemented in a variety of forms. Band-pass frequency of an RF (radio frequency) filter is determined by the filter's inductance and capacitance characteristics, and the operation of adjusting the filter's band-pass frequency is called tuning.
Arbitrary frequency bands are assigned to those doing business with communication systems such as mobile communication systems, and the communication operators divide the assigned frequency bands into several channels for use. In the case of the related art, communication operators produced a filter suited to each frequency band separately for use.
In recent times, however, with the rapid changes in the communication environment, there arose a need for such characteristics as center frequency and bandwidth to vary, unlike the environment in the early years of using mounted filters. A tunable filter is used for varying such characteristics.
FIG. 1 is a drawing illustrating a conventional filter's structure.
Referring to FIG. 1, the conventional filter comprises a housing 110, an input connector 120, an output connector 130, a cover 140, multiple cavities 150, and resonators 160.
Inside the housing 110 there are multiple walls, by which each of the cavities 150 that hold resonators is defined. The cover 140 is equipped with joining holes and tuning bolts 170 for joining the housing 110 and the cover 140.
The tuning bolts 170 are joined to the cover and penetrate into the inside of the housing. The tuning bolts 170 are placed on the cover 140 in positions corresponding to the resonators or to designated positions inside the cavities.
RF signals (or frequency signals) are input to the input connector 120 and output to the output connector 130, and they proceed to the subsequent cavity 150 through the coupling window in each cavity 150. Each of the cavities 150 and resonators 160 cause resonance in the RF signals, and the RF signals are filtered by this resonance phenomenon.
In a conventional filter such as that of FIG. 1, the tuning of frequency characteristics such as center frequency and bandwidth are carried out by way of tuning bolts 170.
FIG. 2 is a cross-sectional view of a cavity in a conventional filter.
Referring to FIG. 2, the tuning bolt 170 goes through the cover 140, and is located above the resonator 160. The tuning bolt 170 is composed of a metal, and is secured to the cover 140 by screw-joining.
Consequently, by rotating the tuning bolt 170, the distance between it and the resonator 160 may be adjusted, and tuning is achieved by varying the distance between the resonator 160 and the tuning bolt 170. The tuning bolt 170 may be rotated manually, or a separate tuning machine may be used for the rotating of the tuning bolt. Once tuning is achieved at a suitable position, the tuning bolt 170 is secured by a nut.
In a conventional filter, as the distance between the tuning bolt 170 and the resonator 160 changes with the rotating of the tuning bolt 170, capacitance also changes. Capacitance is one of the parameters that determine the filter's frequency, and the filter's center frequency changes with the change in capacitance.
In such a conventional filter, tuning was only possible at the early phase of production, its structure making it difficult for tuning during use. To solve such a problem, a tunable filter that enables comparatively easier tuning by a sliding method was proposed.
The sliding-method frequency tunable filter performs tuning of frequency band characteristics such as the filter's resonance frequency and bandwidth by the sliding motion of a sliding part, installed to slide between the cover 140 and the resonator 160, and having a tuning element made of a metallic or a dielectric material attached to its underside. The sliding part may be slid automatically by the use of a motor, or manually by a user.
A tunable filter using such sliding method has the advantage of making tuning possible just by moving the sliding part sideways without the user having to rotate the tuning bolt.
However, even with a tunable filter using a sliding part, a user had to slide the sliding part manually by personally visiting the area where the tunable filter is installed, or manually control the motor for sliding the sliding part, in order to change the filter's frequency characteristics. Consequently, if the tunable filter was installed in a remote area with little accessibility, such as in a mountainous region, there was a problem of the tuning of the filter's frequency characteristics being bothersome and time-consuming